First of all I don't really understand why anyone would buy a pair of headphones they don't like. You are talking about the "plasticky" mids. What exactly do you mean by that? Personally I have done a direct comparison between k501 and k701 and I can assure you that k701 is k501 with more and better defined bass and somewhat airier treble. The mids are as fine as on k501. Either something was wrong with the headphones you tested or there is a mismatch in your system. I drive mine with Matrix M-Stage amp with a Harman HD990 CDP as a source.
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I haven't heard the other AKGs you mention above so can't compare ... but against Senn 650s, beyer DT880-250, the K701 are probably the most balanced WRT freq. spectrum. The HeadRoom graph seems to point to that, too.
BTW, the SQ -- at current 160hrs -- continues to improve, but not as dramatically. (Now if I can work the same magic with my Grado 350i's, which I similarly gave up on a few dozen hrs after new (like 701s ... literally sat idle for years, until I (finally) heard about the extended burn-in time ....)